A panel made of an Al—Mg—Si aluminum alloy sheet of AA or JIS 6000 series (hereinafter simply referred to as 6000-series) as a raw material has a problem that appearance quality defects such as ridging marks are likely to develop in a surface thereof. The ridging marks are a phenomenon of roughness produced in a sheet surface during deformation such as by press forming due to textures arranged in stripes in the sheet. The phenomenon is troublesome because the ridging marks are produced by press forming even when the grains of the aluminum alloy sheet as the raw material are fine enough not to cause surface roughness. There is also a problem that the ridging marks are relatively unnoticeable immediately after press forming and become noticeable after the panel is advanced as a panel structure, without any modification, to a painting step.
The ridging marks are particularly likely to be produced when press forming conditions become stricter due to an increased size of the panel structure, a complicated shape thereof, a thinned thickness thereof, or the like. There is also the problem that the ridging marks are relatively unnoticeable immediately after press forming and become noticeable after the panel is advanced as the panel structure, without any modification, to the painting step.
When the ridging marks are produced in a panel structure for use as an outside panel (outer) of which a good-looking surface is particularly required, a problem arises that the appearance thereof becomes poor, and the panel structure cannot be used.
As an approach to such a problem of the ridging marks, it has been conventionally known to cool an ingot after homogenizing heat treatment at a temperature of 500° C. or higher, or reheat the ingot after being cooled to room temperature, start hot rolling at a relatively low temperature of 350 to 450° C. or control a compound, and thereby prevent ridging marks in an excess-Si 6000-series aluminum alloy sheet (See Patent Documents 1, 2, 3, and 10).
There have also been proposed various methods in which textures (crystal orientations) in a 6000-series aluminum alloy sheet are controlled to improve ridging marks. For example, it has been proposed to focus attention on a crystal orientation component of the {100} plane, and reduce the degree of integration of Cube orientation in a sheet surface layer by 2 to 5, and reduce a grain size in a sheet surface portion to 45 μm or less (see Patent Document 4). It has also been proposed to simultaneously regulate the distribution densities of various orientations such as, e.g., Cube orientation, Goss orientation, Brass orientation, CR orientation, RW orientation, S orientation, and PP orientation in a 6000-series aluminum alloy sheet (see Patent Documents 5 and 9).
It has been further proposed to set the ratio of grain boundaries in which the difference between adjacent crystal orientations is 15° or less to 20% or more (see Patent Document 6). In addition, it has also been proposed to set an earing rate in a 6000-series aluminum alloy sheet to 4% or more, and set a grain size therein to 45 μm or less (see Patent Document 7). It has also been proposed to provide, in an aluminum alloy containing Mg, a specified relationship between the area ratio of grains of which the plane orientation in a surface of the alloy is within a range of 10° from the (100) plane and an area ratio of grains of which the plane orientation in the surface of the alloy is within a range of 20% from the (100) plane (see Patent Document 8).    Patent Document 1: Japanese Patent No. 2823797    Patent Document 2: Japanese Unexamined Patent Application Publication No. Hei 08-232052    Patent Document 3: Japanese Unexamined Patent Application Publication No. Hei 07-228956    Patent Document 4: Japanese Unexamined Patent Application Publication No. Hei 11-189836    Patent Document 5: Japanese Unexamined Patent Application Publication No. Hei 11-236639    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2003-171726    Patent Document 7: Japanese Unexamined Patent Application Publication No. 2000-96175    Patent Document 8: Japanese Unexamined Patent Application Publication No. 2005-146310    Patent Document 9: Japanese Unexamined Patent Application Publication No. 2004-292899    Patent Document 10: Japanese Unexamined Patent Application Publication No. 2005-240113